Real-life objects can look different when viewed from different angles and when illuminated from different directions and/or angles. For example, when facing a low lying sun, a landscape may have a particularly different appearance than when viewing the same landscape at high noon. Painters and photographers often explore the appearance of trees, landscapes, people and urban areas under a variety of conditions, in order to accumulate knowledge about “how things look”.
A reason that things look differently under different conditions can be related to how light is reflected from the surface of viewed objects. Light reflected in one direction may make an object appear different than when it is reflected in a different direction. Further, structural and optical properties of the surface of viewed objects, such as shadow-casting, multiple scattering, mutual shadowing, transmission, reflection, absorption and emission by surface elements, can also affect how an object appears. These characteristics of an object can be summarized by the reflectance on real-world objects, and the reflectance of a surface can be determined by a Bidirectional Reflectance Distribution Function (BRDF). Determining BRDFs, for example, can be useful for creating realistic computer-generated (CG) images.